Field of the Invention
This invention concerns compounds useful as an additive in lubricants and greases for friction reduction, wear reduction, and/or extreme pressure performance, among other applications described in detail in this application.
Described herein is the development of highly sulfurized binuclear molybdate salts with application as additives in lubricants. This class of compounds may be represented by the following formula:
where a molybdenum salt is prepared which comprises two countercations (Q1 and Q2) and a binuclear sulfur-containing molybdate anion (Y).
Discussion of and Comparison with Related Art
This invention involves the preparation and application of imidazolium oxothiomolybdate salts related to a class of compounds described in Coucouvanis et al. (Inorg. Chem. 1988, 27, 3272-3273). The compounds described herein are useful as additives in lubricants for friction reduction, wear reduction, and extreme pressure performance. Coucouvanis et al. teach synthesis of thiomolybdenyl complexes with [Mo2S2O2]2+ cores and substitutionally labile ligands. Unlike the quaternary ammonium salts prepared in Coucouvanis et al., the compounds described in this application use imidazolium countercations which greatly influence physical properties and performance of the compounds. In all cases reported, the imidazolium oxothiomolybdate salts are room-temperature ionic liquids.
According to U.S. Pat. No. 4,370,245, certain tetrahydrocarbylammonium thiomolybdate containing at least about 15 carbon atoms, such as trioctylmethylammonium thiomolybdate, enhances the extreme pressure properties of substituted-thickened urea greases. The compound used in the invention described herein differs from the compound of the '245 Patent in that the core of the sulfur-containing molybdate structure is distinct from thiomolybdate (MoS4)2−. The molybdenum core of the class of compounds described herein is binuclear with respect to molybdenum and contains oxygen and/or sulfur. Also, the countercations for the described invention are imidazolium-based rather than quaternary ammonium countercations.
U.S. Pat. No. 3,356,702 describes a class of dithiocarbamates having the general formula MoO2(SCSNR2)2. The compound used in the invention described herein differs from the compound of the '702 Patent in that the compounds described herein are molybdenum-containing salts rather than neutral organometallic compounds. In addition, the molybdenum-containing salt has higher sulfur to molybdenum ratio than the molybdenum dithiocarbamate technology.
U.S. Patent Application Publication No. 2014/0171348 teaches improving solubility of an ionic liquid in a lubricating oil by using as the lubricating oil a formulated oil including a lubricating oil base stock as a major component and an ionic liquid imidazolium salt base stock as a minor component, which ionic liquid imidazolium salts are represented by the formula:

The compound used in the invention described herein differs from the compound of US 2014/0171348 in that the class of compounds described herein involves a molybdenum-containing anion as the counterion for imidazolium-based lubricant additives.
This class of additives improves upon current technology such as molybdenum dithiocarbamates and molybdenum disulfide by increasing the sulfur to molybdenum ratio. These high-sulfur containing additives can exhibit good performance in terms of friction reduction, wear reduction, and/or extreme pressure properties. Furthermore, use of imidazolium as countercations to the oxothiomolybdate dianions results in products that are predominantly room-temperature ionic liquids. Such compounds may have applications beyond lubricants and greases. For example, these compounds can have applications in areas including but not limited to polymer additives, paint additives, specialized solvents and lubricants, refinery chemicals, dissolution and transport of reactive gases (US 2006060818, US 2006060817), metal plating processes (U.S. Pat. No. 4,446,331, U.S. Pat. No. 4,446,350, U.S. Pat. No. 4,446,349), electropolishing of steel surfaces (US 20040097755), antistatic cleaning agents for surfaces, surfactant technologies (WO 2006111712), catalysts and co-catalysts, electrochemical devices, electrolytes for solar cells (U.S. Pat. No. 5,350,644), fuel cells, batteries, high shear mixing technologies (US 20050119423), and extraction (most notably the biphasic acid scavenging using ionic liquids “BASIL™” process developed by BASF, —US 20040073035, US 20050020857, US 20080083606) and separation (US 20040133058) processes. In addition, a variety of ionic liquids have been used industrially as solvents and additives for numerous reactions in organic synthesis that include but are not limited to the nickel-catalyzed dimerization and oligomerization of alkenes (J. Mol. Catal, 1994, 92, 155-164; US 20050113621), hydrosilylation of alkenes, isomerization processes (U.S. Pat. No. 5,238,889), transition metal-catalyzed cross-coupling reactions, olefin metathesis reactions, carbonylation catalysis (U.S. Pat. No. 6,320,083), electrochemical oxidation of sulfur-containing compounds in naphtha (US 20026338788), alkylation reactions (US 20060135839, US 20040133056), and catalytic hydroformylation (US 20070161829). The unique combination of two ionic materials (imidazolium cations and molybdenum-containing anions) to form a liquid product has significant potential in many future industrial applications. Notably, a few examples have been reported in which ionic liquids have been prepared with mononuclear molybdenum-containing anions for use in organic synthesis in the reduction of sulfoxides (Tetrahedron Lett., 2013, 54, 3765-3768; New J. Chem., 2012, 36, 971-976), desulfurization of natural gasoline (Mol. Divers., 2010, 14,777-787), and oxidation of alcohols (Adv. Synth. Catal., 2005, 347, 231-234).
The preparation methods of the molybdenum-containing salts described in Coucouvanis et al. and Recatalá et al. (Dalton Trans., 2013, 42, 12947-12955) were adapted by the inventors of this application for the preparation of imidazolium sulfur-containing molybdate ionic liquids. The reported procedure was further modified to improve removal of unreacted elemental sulfur. The preparation methods described herein remove excess sulfur from the reaction mixture with an appropriate solvent (i.e. acetonitrile) and avoid a recrystallization step. The extraction solvent and unreacted sulfur can be separated by distillation and recycled in the process.